It is known that collimation of x-rays results in an improvement in the image quality of an x-ray imaging system. This is because the collimation of an x-ray source reduces the amount of scattered x-ray photons which reach the x-ray sensor elements, after having passed through the subject matter being imaged. These scattered x-ray photons would otherwise contribute to the reading from the sensor elements and reduce the overall contrast in the x-ray image because they do not convey the same relevant diagnostic information as the unscattered x-ray photons that have passed directly from the x-ray source to the sensor element. Scattered photons are responsible for the haze often associated with radiographs.
Generally, x-ray collimators have comprised a two dimensional grid, sometimes also known as an anti-scatter grid (ASG), which is positioned directly in front of the sensor and serves to absorb or block photons emanating with a large angle. These ASGs are often grid structures composed of high density metals whose operation can be considered analogous to a venetian blind collimating optical photons. A variety of geometries and fabrication methods have been described in the literature, each with the similar goal of reducing the unwanted scattered photons from impinging upon the sensor.
In addition to anti-scatter methods, x-ray lenses have been considered. A wide range of approaches has been discussed in an attempt to focus x-rays with more efficiency or better focal properties. Examples of x-ray lenses include, polycapillaries (assembled and fused) and Wolter Optics (a grid of materials) both of which essentially work by collectively reflecting a single source of x-ray photons. Refractive lenses have also been described.
In recent years there have been advances in the development of micrometer scale x-ray sources, such that it is now possible to produce a two dimensional array of x-ray sources with a typical distance between the x-ray sources of the order of 100 μm to 1 cm or more.
An example of such a two-dimensional x-ray source is provided in WO 2011/017645 apparatus for producing x-rays for use in imaging.
Known collimation and lensing methods are not so useful for collimating a two dimensional array of x-ray sources and it is an aim of embodiments of the present invention to at least partially mitigate the disadvantages of known x-ray collimation methods and to provide a means of collimating x-rays emanating from a two-dimensional array of x-ray sources.